The present invention relates to a phosphor which emits light under X ray excitation and an X-ray intensifying screen using the phosphor, which screen being especially used for medical and industrial radiography.
An X-ray excited phosphor is used mainly for X-ray intensifying screen. This X-ray intensifying screen is generally used in conjunction with photographic film for the purpose of improving the sensitivity of the photographic system in medical radiography. The phosphor used for such an X ray intensifying screen is required to have good X-ray absorption, a high X-ray to-light conversion efficiency, and a low afterglow noise.
An X-ray intensifying screen coated with phosphor of good X-ray absorption has higher definition of X-ray image and therefore can improve the diagnosis efficiency in the medical radiography. A phosphor having a high X-ray-to-light conversion efficiency can be used with only a little quantity of X-ray irradiation, and thereby, the exposure dose of the subject can be reduced. Further, a phosphor having a low quantity of afterglow component can prevent errors in consultation caused by residue (afterglow noise).
Recently, for the purpose of reducing the exposure dose of the subjects, X ray intensifying screens using a phosphor containing Gd.sub.2 O.sub.2 S:Tb, BaFCl:Eu, LaOBr:Tm, YTaO.sub.4 :Tm or the like, instead of conventional phosphor of CaWO.sub.4 have been put to practical use.
However, since BaFCl:Eu and LaOBr:Tm have low X-ray absorption dose, the definition of the obtained X-ray photograph is not good. Further, since the phosphor is shaped into flat granules, light emitted by the excitation of X-ray is much scattered and the sharpness of the X-ray image becomes low.
A phosphor containing Gd.sub.2 O.sub.2 S:Tb emits light in the range from the blue zone to the green zone, and is used in combination with an orthochromatic film which is sensitive in the range from the blue zone to the green zone. Therefore, the film is easily exposed to light even in the darkroom and the darkroom lamp is required to be dim, which causes low workability.
Rare earth tantalate phosphors exhibit undesirable delayed fluorescence(afterglow), and afterglow noises occur when the phosphor screen is used in rapid succession for several films which limits its use. If a phosphor is developed which has a low afterglow noise and is provided with the advantages of rare earth tantalate phosphors, an ideal phosphor for X-ray use can be realized.
An important object of the present invention is to provide a a phosphor which can emit light by the excitation of X-ray, can absorb a large quantity of X-ray and has a high X-ray-to-light conversion efficiency and a low afterglow noise.
Further, another important object of the present invention is to provide an X-ray intensifying screen which affords to take a radiograph with only a small exposure dose of a subject and permits one to make a correct diagnosis of the subject.
The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawing. It is to be expressly understood, however, that the drawing is for purpose of illustration only and is not intended as a definition of the limits of the invention.